Primary extruder

The secondary extruder is typically larger in diameter than the primary machine, and it rotates at a lower screw speed. For example, a midsized tandem line would be constructed using a 114.3 mm diameter primary extruder and a 152.4 mm secondary extruder. The primary extruder would operate at a normal screw speed (50 to 70 rpm) and could generate the pressure required to run the downstream equipment. The secondary extruder would operate at a lower screw speed (10 to 15 rpm) so that the entry and exit pressure for the extruder are about the same. 

Cooling water flow to the barrel jacket coolers for the secondary extruder removes the excess energy in the resin dissipated by the primary extruder and also the energy dissipated by the screw in the secondary machine. For a properly designed line, the performance of the secondary extruder determines the overall rate of the process. 

Fig. 11 Foamed sheet extrusion line. 1 Primary extruder, 2 blowing agent metering and flow control, 3 screen pack/breaker plate, 4 secondary extruder, 5 annular die, 6 cooling mandrel, 7 guide rolls, 8 winders. [38]...

Insulation, Cross-linked polyethylene (XPLE) and ethylene—propylene mbber (EPR), both thermosets, are the primary extruded dielectrics used in medium and high voltage power cables (Table 4). 

Polystyrene foam sheet can be made by a variety of extrusion processes (single screw, twin screw) but is most commonly made using a tandem extrusion process as shown in Figure 11.6. This process uses two extruders in series, which effectively optimize the melting, mixing, and cooling unit operations. The diameters of the extruders currently used in this process are usually 115 mm for the primary extruder and 150 mm for the secondary extruder, with an output capability of 400 kg/h. 

Figure 11.6 A tandem extrusion process for the manufacture of polystyrene foam sheet. (1) Primary extruder (2) blowing agent addition system (3) screen changer (4) secondary extruder (5) annular die (6) cooling mandrel (7) S-wrap (8) winders...

In downstream, the pellets are sent to primary extruder. Mixing is done at 20 rpm. The temperatures from feed zone to die head are 180°C, 240°C, 240 C and 260°C. These mixed samples are then sent to a secondary extruder with a screw rotation speed of 90 rpm and the pressure adjustable die temperature is 220°C. The supercritical fluid (CO gas) is compressed at 21 MPa pressure using a positive displacement syringe pump and sent to the secondary extruder. The melt and supercritical fluid are mixed with the help of a distributive-type screw. This mixture is pumped to the pressure adjustable die, where the sample is casted and the supercritical fluid escapes out. A similar fabrication route has been used elsewhere [43,54-55]. 

Quench. Cooling the melt to solidily the viscous extrudate is an important process parameter. Depending on the orientation process, the melt quenching process may be very different. The two primary extrudate geometries are a flat sheet and a tube. Most often, heat needs to be removed from the extrudate as quickly as possible. For semicrystalline polymers with a relatively slow crystallization kinetics, like PET, PEEK, or PPS, rapidly quenching the melt results in a molecular amorphous state. For semicrystalline polymers with a relatively fast crystallization kinetics, like polyethylene, polypropylene, or nylon 6, rapidly quenching the melt allows for the growth of smaller spherulites. 

The model of morphology formation during the doublebubble process is presented on Fig.l After extrusion the primary extruded tube is quenched with water in order to prevent big crystallite growth and then reheated to temperatures below melting point. Before the stretching some residual crystallinity can be observed in the material... 

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